linux-kernel - Re: [PATCH 11/15] sched: Pass unlimited __cpu_power information to upper domain level groups

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Message-ID: <20090824164452.GR29572@balbir.in.ibm.com>
Date:	Mon, 24 Aug 2009 22:14:52 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andreas Herrmann <andreas.herrmann3@....com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Gautham Shenoy <ego@...ibm.com>,
	"svaidy@...ux.vnet.ibm.com" <svaidy@...ux.vnet.ibm.com>
Subject: Re: [PATCH 11/15] sched: Pass unlimited __cpu_power information to
	upper domain level groups

* Peter Zijlstra <peterz@...radead.org> [2009-08-24 17:21:37]:

> On Thu, 2009-08-20 at 15:41 +0200, Andreas Herrmann wrote:
> > For performance reasons __cpu_power in a sched_group might be limited
> > such that the group can handle only one task. To correctly calculate
> > the capacity in upper domain level groups the unlimited power
> > information is required. This patch stores unlimited __cpu_power
> > information in sched_groups.orig_power and uses this when calculating
> > __cpu_power in upper domain level groups.
> 
> OK, so this tries to fix the cpu_power wreckage?
> 
> ok, so let me try this with an example:
> 
> 
> Suppose we have a dual-core with shared cache and SMT
> 
>   0-3     MC
> 0-1 2-3   SMT
> 
> Then both levels fancy setting SHARED_RESOURCES and both levels end up
> normalizing the cpu_power to 1, so when we unplug cpu 2, load-balancing
> gets all screwy because the whole system doesn't get normalized
> properly.
> 
> What you propose here is every time we muck with cpu_power we keep the
> real stuff in orig_power and use that to compute the level above.
> 
> Except you don't use it in the load-balancer proper, so normalization is
> still hosed.
> 
> Its a creative solution, but I'd rather see cpu_power returned to a
> straight sum of actual power to normalize the inter-cpu runqueue weights
> and do the placement decision using something else.

The real solution is to find a way to solve asymmetric load balancing,
I suppose. The asymmetry might be due to cores being hot-plugged for
example

-- 
	Balbir
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